LT1932 Linear Technology, LT1932 Datasheet - Page 6

• Manufacturer Part Number: LT1932
• Description: Constant-Current/ Constant-Voltage 1.4MHz Step-Up DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1932
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple
voltage. Because they have an extremely low ESR and are
available in very small packages, multilayer ceramic ca-
pacitors are an excellent choice. X5R and X7R type
capacitors are preferred because they retain their capaci-
tance over wider voltage and temperature ranges than
other types such as Y5V or Z5U. A 1 F or 2.2 F output
capacitor is sufficient for most applications. Always use a
capacitor with a sufficient voltage rating. Ceramic capaci-
tors do not need to be derated (do not buy a capacitor with
a rating twice what your application needs). A 16V ce-
ramic capacitor is good to more than 16V, unlike a 16V
tantalum, which may be good to only 8V when used in
certain applications. Low profile ceramic capacitors with
a 1mm maximum thickness are available for designs
having strict height requirements.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1932. A 2.2 F or 4.7 F input capacitor
is sufficient for most applications. Table 2 shows a list of
several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
VENDOR
Taiyo Yuden
Murata
Kemet
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are the ideal choice for LT1932
applications. Table 3 shows several different Schottky
diodes that work well with the LT1932. Make sure that the
diode has a voltage rating greater than the output voltage.
The diode conducts current only when the power switch is
6
PHONE
[phone]
[phone]
[phone]
U
U
W
URL
www.t-yuden.com
www.murata.com
www.kemet.com
U
turned off (typically less than one-third the time), so a 0.4A
or 0.5A diode will be sufficient for most designs.
Table 3. Recommended Schottky Diodes
PART
MBR0520
MBR0530
MBR0540
ZHCS400
ZHCS500
Programming LED Current
The LED current is programmed with a single resistor
connected to the R
internally regulated to 100mV, which sets the current
flowing out of this pin, I
LT1932 regulates the current into the LED pin, I
times the value of I
better) resistor value should be used. Table 4 shows
several typical 1% R
values, use the following equation to choose R
Table 4. R
Most white LEDs are driven at maximum currents of 15mA
to 20mA. Some higher power designs will use two parallel
strings of LEDs for greater light output, resulting in 30mA
to 40mA (two strings of 15mA to 20mA) flowing into the
LED pin.
R
SET
SET
I
225
LED
Resistor Values
40
30
20
15
10
5
(mA)
•
0 1
I
LED
SET
.
SET
V
SET
pin (see Figure 1). The R
. For the best accuracy, a 1% (or
SET
values. For other LED current
, equal to 100mV/R
VENDOR
ON Semiconductor
[phone]
www.onsemi.com
Zetex
[phone]
www.zetex.com
R
SET
562
750
1.13k
1.50k
2.26k
4.53k
VALUE
LED
SET
SET
SET
, to 225
.
pin is
. The